docs: def label_durations(self, label_list_ids=None):
          """
          Return a dictionary containing the total duration, every label-value in this corpus is occurring.
  
          Args:
              label_list_ids (list): If not None, only labels from label-lists with an id contained in this list
                                     are considered.
  
          Returns:
              dict: A dictionary containing the total duration with the label-value as key.
          """
          duration = collections.defaultdict(int)
  
          for utterance in self.utterances.values():
              for label_value, utt_count in utterance.label_total_duration(label_list_ids=label_list_ids).items():
                  duration[label_value] += utt_count
  
          return duration